1. Field of the Invention
The present invention relates to a power supply apparatus driving circuit, a power supply apparatus driving integrated circuit, and a power supply apparatus, which include a low-side switching element and a high-side switching element with different ground potential levels in a switching power supply apparatus.
2. Description of the Related Art
For example, Japanese Unexamined Patent Application Publication No. 7-274498 discloses a power supply apparatus that includes a low-side switching element and a high-side switching element with different ground potential levels, and drives the low-side switching element and the high-side switching element by means of a control IC.
FIG. 1 is a circuit diagram of the power supply apparatus exemplified as related art in Japanese Unexamined Patent Application Publication No. 7-274498. This power supply apparatus includes a DC power supply 1, first and second field effect transistors (FETs) 2 and 3, an output transformer 4, a resonant circuit 6, a load 8, and a control circuit 9. The first and second FETs 2 and 3 respectively serve as first and second switching elements that are connected between one end and the other end of the DC power supply 1. The output transformer 4 has a primary winding 4a, a secondary winding 4c, and a tertiary winding 4b. The resonant circuit 6 includes the primary winding 4a of the output transformer 4 which is connected in series with each of the first and second FETs 2 and 3, a resonant capacitor 5, and a resonance reactor configured by an inductance that is formed integrally with the primary winding 4a. The load 8 is connected to the secondary winding 4c of the output transformer 4 via a rectifier smoothing circuit 7. The control circuit 9 operates from the voltage across the tertiary winding 4b of the output transformer 4, and supplies a control signal to the gate terminal (control terminal) of each of the first and second FETs 2 and 3 to control on/off of the first and second FETs.
The rectifier smoothing circuit 7 includes diodes 10, 11, and a smoothing capacitor 12. A voltage induced in the second primary winding 4b of the output transformer 4 is supplied to a power supply terminal 9a of the control circuit 9 via a rectifying circuit 13 and a smoothing capacitor 14. A signal of the output voltage to the load 8 is fed back to a current sensing element 9b of the control circuit 9 via an operational amplifier 17, a reference voltage source 18, photo-couplers 15, 16, and a limiting resistor 61.
In the related art, as illustrated in FIG. 1, the power supply apparatus that drives the low-side switching element and the high-side switching element by means of the control IC is configured so that, at startup, driving voltage is supplied to the control circuit via a starting resistor 60 from a high voltage that is the input voltage of the switching power supply, and during normal operation, voltage is supplied from the transformer.
However, current continues to flow through the starting resistor 60 even after startup, thus generating loss. A conceivable way to eliminate the loss due to the starting resistor is to provide a switching circuit that disconnects the starting resistor and the control circuit from each other. However, the switching circuit is required to have high breakdown voltage. Accordingly, the entire control circuitry including the control circuit and the switching circuit needs to be designed with high breakdown voltage.
With such a configuration, the control circuit differs for each converter system, which limits the number of power supply apparatuses that can be used. Also, high breakdown voltage designs are required. For such reasons, when constructing an integrated control circuit, the resulting control IC becomes very expensive.
To address this problem, it is effective to configure the control circuit portion of the converter by two control ICs, by dividing the control circuit portion into a drive circuit for driving the FETs, and a control signal generating circuit for generating pulse waveforms that serve as control signals. In this case, the control signal generating circuit portion is configured by a low breakdown voltage process to thereby reduce cost, and for the driving circuit that requires a high breakdown voltage process, cost can be reduced by making different converter systems common and hence increasing the number of power supply apparatuses that can be used. As a result, it is possible to realize a low-cost control circuit, even through the control circuit is configured by two control ICs.
However, in the above related art, the driving circuit configured by a high breakdown voltage process requires another power supply apparatus to supply the driving voltage for the switching elements, and it is necessary to provide another power supply apparatus to start and operate the main converter. Therefore, it is not possible to miniaturize the power supply apparatus as a whole.